Milling system and method of milling

ABSTRACT

A mill assembly includes a shaft; a lead mill secured to a first end of the shaft, the lead mill including a first body having a plurality of first blades and a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades; and a second mill secured to the shaft a selected distance from the lead mill, the second mill including a second body having a plurality of second blades and a plurality of second cutters having substantially cylindrical bodies secured to the plurality of blades.

BACKGROUND OF INVENTION

1. Field of the Invention

Embodiments disclosed herein relate generally to a downhole mill assembly. More particularly, the embodiments disclosed herein relate to a method of milling and a method of designing a mill assembly.

2. Background Art

When an existing cased oil well becomes unproductive, the well may be sidetracked in order to develop multiple production zones or redirect exploration away from an unproductive zone. Generally, sidetracking involves the creation of a window in the well casing by milling the steel casing in an area either near the bottom or within a serviceable portion of the well. The milling operation is then followed by the directional drilling of rock formation through the newly formed casing window. Sidetracking enables the development of a new borehole directionally oriented toward productive hydrocarbon sites without moving the rig, platform superstructure, or other above ground hole boring equipment, and also takes advantage of a common portion of the existing casing and cementing in the original borehole.

Thus, sidetracking is often preferred because drilling, casing, and cementing the borehole are avoided. As mentioned above, this drilling procedure is generally accomplished by either milling out an entire section of casing followed by drilling through the side of the now exposed borehole, or by milling through the side of the casing with a mill that is guided by a wedge or “whipstock” component.

The casing window is generally created with a combination of mills mounted on a shaft or mandrel at the bottom end of a drill string and wedging between the casing and a whipstock, which is generally set in the hole in combination with the first milling run.

The peripheral surface of mills is generally covered with abrasive or cutting inserts made of hard material such as sintered tungsten carbide compounds brazed on a steel shaft. The hardness of the whipstock is generally designed so that minimum wear will be generated by the rotation of mills peripheral surface onto the whipstock face while the assembly is pushed and rotated against the casing wall under deflecting action of the whipstock. The milling action generally results from unbalanced pressures between the mill(s) and the whipstock on one hand and the mill(s) and the casing wall on the other hand.

U.S. Pat. No. 4,266,621, which is herein incorporated by reference, describes a milling tool for elongating a laterally directed window in a well casing. The disclosed system requires three trips into the well, beginning with the creation of an initial window in the borehole casing, the extension of the initial window within a particular cutting tool, and the elongation and further extension of the window by employing an assembly with multiple mills. While the window mill is aggressive in opening a window in the casing, the number of trips, typically three, to accomplish the task is expensive and time consuming.

By integrating a whipstock into the milling operation and directionally orienting the milling operation to a more confined area of well casing, the number of trips required to effectively mill a window in a well casing has been decreased. A whipstock having an acutely angled ramp is first anchored inside a well and properly oriented to direct a drill string in the appropriate direction. A second trip is required to actually begin the milling operations. Newer methods integrate the whipstock with the milling assembly to provide a combination whipstock and staged sidetrack mill, allowing for casing windows to be milled in one trip. The milling assembly is connected at its leading tool to the top portion of the whipstock by a bolt, which upon application of sufficient pressure, may be sheared off to free the milling assembly. The cutting tool employed to mill through the metal casing of the borehole has conventionally incorporated cutters that include at least one material layer, such as preformed or crushed tungsten carbide, designed to mill pipe casing. Several such one-trip milling systems include those described in U.S. Pat. Nos. 5,771,972, 6,102,123, 6,648,068, which are herein incorporated by reference in their entirety.

Conventional milling systems are, however, unable to mill windows in chrome casings, casings which are steadily increasing in number of wells due to the number of wells in severe drilling environments, such as severely corrosive environments, deep wells, cold environments, and sea bottoms, that are more commonly drilled due to exhaustion of easily drillable wells. Severe environmental conditions typically include the presence of corrosive fluids with dissolved gases including oxygen (O₂), hydrogen sulfide (H₂S), and carbon dioxide (CO₂) gasses. Due to the exposure to the severely corrosive environments, many downhole components are exposed to a variety of corrosion mechanisms such as uniform corrosion, pitting, corrosion fatigue, sulfide stress cracking, hydrogen blistering, hydrogen embrittlement, stepwise cracking, wormhole attack, galvanic ringworm corrosion, heat affected corrosion, mesa attack, raindrop corrosion, and erosion corrosion, necessitating the use of corrosion resistant alloys (CRA), frequently duplex chrome, in the downhole components including casings. Other typically corrosion and/or erosion resistant CRA-type materials include: (1) stainless steel including conventional austenitic, martensitic, precipitation hardened, duplex, and ferritic stainless steels; (2) precipitation hardened and solid solution nickel-based alloys and nickel copper alloys; and (3) cobalt-based, titanium, and zirconium alloys.

In a duplex 25% chrome casing, for example, although desired corrosion resistance can be obtained, the material proves to be difficult in handling, specifically, in cutting and machining. The material tends to be abrasive to cutting tools, as well as leading to work hardening, smearing, galling, and welding. Furthermore, in cutting chips in a chrome casing, high asperity-junction temperatures are frequent.

The difficulties associated with milling through a chrome casing leaves many mature wells neighbored by significant quantities of oil otherwise unreachable without the cost of either pulling the chrome casings and recompleting the existing well or forming a new well. The ability to sidetrack a well would not only allow for a multilateral well, but would also allow for sidetracking of a stuck tubular.

As reported in “A New Casing-Exit System for Duplex 25% Chrome Casing Enables Economically Viable Redevelopment of Mature Fields,” OTC 17594 (2005), a specially designed milling system, requiring a new motor design to avoid over-torque on the drill assembly, was the first reported milling system able to cut an exit in a chrome well. However, further improvements in milling systems would allow for increased longevity of mills in an operational environment that frequently leads to failure of mills by abrasion and/or galling.

SUMMARY OF INVENTION

In one aspect, embodiments disclosed herein relate to a mill assembly that includes a shaft; a lead mill secured to a first end of the shaft, the lead mill including a first body having a plurality of first blades and a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades; and a second mill secured to the shaft a selected distance from the lead mill, the second mill including a second body having a plurality of second blades and a plurality of second cutters having substantially cylindrical bodies secured to the plurality of blades.

In another aspect, embodiments disclosed herein relate to a method of milling a window in a tubular in a wellbore that includes engaging a lead mill of a mill assembly against an interior surface of the tubular, the lead mill secured to an end of a shaft and including a first body having a plurality of first blades and a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades; rotating the mill assembly; moving the mill assembly along a surface of a whipstock assembly as the lead mill cuts the window in the tubular, thereby deflecting the lead mill and shaft outwardly through the window in the tubular; and engaging a second mill of the mill assembly against the window in the tubular, the second mill secured to the shaft a selected distance from the lead mill and including a second body having a plurality of second blades and a plurality of second cutters having substantially cylindrical bodies secured to the plurality of blades.

In another aspect, embodiments disclosed herein relate to a method of designing a mill assembly that includes determining characteristics of a lead mill, the lead mill including a first body having a plurality of first blades and a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades; determining characteristics of a shaft having a first and second end, wherein the first end is adapted to receive the lead mill and the second end is adapted to be threadably connected to a drill assembly; determining characteristics of an engagement point; and selecting a location on the shaft for the engagement point to be placed.

In yet another aspect, embodiments disclosed herein relate to a mill assembly, that includes a shaft; a lead mill secured to a first end of the shaft, the lead mill including a body having a plurality of blades and a plurality of cutters having substantially cylindrical bodies secured to the plurality of blades; and a protective coating disposed on at least a portion of at least one of the body, the plurality of blades, and the plurality of cutters.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a mill assembly according to one embodiment disclosed herein.

FIGS. 2A and 2B show an enlarged side and top view of the lead mill shown in FIG. 1.

FIG. 3 shows an enlarged side view of the second mill shown in FIG. 1.

FIG. 4 shows a mill assembly according to one embodiment disclosed herein

FIG. 5 shows a side view of a cutter to illustrate workface angle.

FIG. 6 shows a side view of a cutter to illustrate rake angle.

FIGS. 7A and 7B shows a cutter according to one embodiment disclosed herein.

FIGS. 8 and 8A show a cutter according to one embodiment disclosed herein.

FIGS. 9A and 9B show a milling and whipstock system which may incorporate an embodiment of a mill assembly disclosed herein.

DETAILED DESCRIPTION

In one aspect, embodiments disclosed herein relate to mill assemblies that include a lead mill, a second mill, and a shank therebetween on which each of lead mill and second mill are attached. Embodiments disclosed herein may also relate to mill assemblies having a single mill attached to the end of a shank, and mill assemblies having a lead mill, one or more second mills, and a shank therebetween. Further, embodiments disclosed herein may also relate to methods of designing a mill assembly, and methods of milling a window in a tubular. As used herein, “second mill” refers to any type of mill, e.g., dress mill, watermelon mill, string mill, follow mill, etc., that may elongate and/or dress the window to full gage.

Referring now to FIG. 1, a mill assembly generally designated as 100 is shown. Mill assembly 100 includes a lead mill 110, which is attached to the bottom end of shaft 120. Located above and spaced a distance X from the lead mill 110 is a second mill 130 that is also mounted on shaft 120. Shaft 120, as shown in FIG. 1, includes a lower section between the lead mill 110 and second mill 130 and an upper section, above the second mill 130. The upper end of shaft 120 may be either threadably connected to a drill string (not shown) or threaded to another subassembly (not shown). As shown in FIG. 1, lead mill 110 has a gage 111 and second mill 130 has a gage 131 such that the gage 111 of lead mill 110 is substantially the same as the gage 131 of second mill 130. One of ordinary skill in the art would recognize that in alternative embodiments, the gage of the second mill 130 may be either larger or smaller than the gage of lead mill 110.

Referring to FIGS. 2A and 2B, the lead mill 110 illustrated in FIG. 1 is shown. Lead mill 110 includes a body 112 and a plurality of blades 114 extending radially from the body 112. The body 112 may be formed from steel or a tungsten carbide matrix infiltrated with a binder alloy or any other material used in the art. A plurality of cylindrically bodied cutters 116 are attached to each of the plurality of blades 114 in cutter pockets 115, typically by brazing. The blades 114 and the cutters 116 generally form a cutting structure of the lead mill 110. Within the mill body 112 are one or more passages ending in openings 118 through which drilling fluid may be delivered to cool the tool surface and remove accumulated debris. Lead mill 110 may also include a threaded connection (not shown) for attachment to the shaft 120 shown in FIG. 1A. After threadedly engaging the lead mill 110 to the shaft 120, the connection may also be welded as known in the art.

Referring to FIG. 3, the second mill 130 illustrated in FIG. 1 is shown. Second mill 130 includes a body 132 and a plurality of blades 134 extending radially from the body 132. The body 132 may be formed from steel or a tungsten carbide matrix infiltrated with a binder alloy or any other material used in the art. A plurality of cylindrically bodied cutters 136 are attached to each of the plurality of blades 134 in cutter pockets 135, typically by brazing. The blades 134 and the cutters 136 generally form a cutting structure of the second mill 130. In the embodiment shown in FIGS. 1-2, second mill 130 is mounted on the shaft 120 via connections (e.g. threaded connections) (not shown). Alternatively, the lead mill 110 and/or second mill 130, including the body 112, 132 and blades 114, 134, may be integral with the shaft 120. In one embodiment, lead mill and/or second mill may be formed from a solid body having integral flow paths formed, for example, by machining, formed therein. In another embodiment, lead mill and/or second mill may be formed from a mold via an infiltration or casting process.

Referring to FIG. 4, a mill assembly generally designated as 200 is shown. Mill assembly 200 includes a lead mill 210, which is attached to the bottom end of shaft 220. Located above and spaced a distance X from the lead mill 210 is a second mill 230 that is also mounted on shaft 220. Shaft 220, as shown in FIG. 1, includes a lower section between the lead mill 210 and second mill 230 and an upper section, above the second mill 230. The upper end of shaft 220 may be either threadably connected to a drill string (not shown) or threaded to another subassembly (not shown). As shown in FIG. 4, second mill 230 is integral with shaft 220. In a particular embodiment, lead mill and/or second mill may be formed from a solid body having integral flow paths formed, for example, by machining, formed therein.

As shown in FIGS. 1-3, the blades 114, 134 are spiral blades positioned about the perimeter of the bodies 112, 132 at substantially equal angular intervals. However, other blade arrangements may be used with embodiments of the present disclosed, and the embodiment shown in FIGS. 1-3 is not intended to limit the scope of the embodiments disclosed herein. For example, the blades 114, 134 may be positioned at unequal angular intervals or be straight instead of spiral.

As also shown in FIGS. 1-3, the mill assembly includes a lead mill and a second mill. However, one of ordinary skill in the art would recognize that in some embodiments, a mill assembly may include multiple second mills, i.e., a third mill, etc., or in other embodiments a mill assembly may instead include a lead mill followed by a motor or stabilizer attached to the shaft a selected distance above the lead mill, rather than a second mill. Additionally, while lead mill 110 and second mill 130 are shown as having cutters 116 and 136 disposed thereon, in some embodiments, either lead mill 110 and/or second mill 130 may not contain cutters attached thereto. In an alternate embodiment, either lead mill 110 and/or second mill 130 may include, instead of cutters, crushed carbide welded thereon.

The cutters 116, 136 attached to lead mill 110 and second mill 130, according to some embodiments disclosed herein, may include tungsten carbide particles and a metal binder. Typical types of tungsten carbide include cemented tungsten carbide (crushed and spherical), cast tungsten carbide (crushed and spherical), macrocrystalline tungsten carbide, and carburized tungsten carbide. In a particular embodiment, the cutters 116, 136 may include crushed cemented tungsten carbide. For various embodiments using cemented tungsten carbide, the cemented tungsten carbide formed from carbide particles ranging in size from about less than 1 to 15 microns and cobalt in amount ranging from about 6 to 16 percent by weight. However, such sizes and amounts are not intended to be a limitation on the scope of the present invention. One skilled in the art would recognize that in using a cemented tungsten carbide, by varying the carbide particle size and/or cobalt content, the wear resistance/hardness and fracture toughness of the cutters may be optimized for a particular milling operation.

In a particular embodiment, the cutters 116, 136 may be formed from crushed cemented tungsten carbide particles ranging in size from about less than 1 to 10 microns and cobalt in amount ranging from 8 to 14 percent by weight. In various other embodiments, the cutters may also include other particles such as, for example, tantalum carbide, tantalum niobium carbide, titanium carbide, tungsten titanium carbide, and tungsten tantalum carbide.

In one embodiment, the cylindrically bodied cutters may be attached to each of the lead mill and the second mill in such a manner so as to provide for a desired workface angle between the cutting face of the cutters and the workface (material being cut) as the cutters engage the casing. The workface angle, shown as 13 in FIG. 5, may be defined as the angle subtended between a plane 520 of the cutting face 515 of the cutter 510 and a line 525 perpendicular to the contact point at the workface surface 530. In a particular embodiment, at least some of the cutters on the lead mill have a workface angle ranging from about −5 to −40 degrees, from about −10 to −35 degrees in another embodiment, from about −15 to −18 in another embodiment, and a workface angle of about −15 degrees in yet another embodiment.

The desired workface angle may be achieved, for example, by varying the cutter geometry or the placement of the cutters in cutter pockets in the blades of a mill to achieve a particular rake angle as known in the art. In one embodiment, the desired workface angle may be achieved by placing cylindrical cutters in angled cutter pockets. In another embodiment, the desired workface angle may be achieved by forming cylindrical bodied cutters having a cutting face angled with respect to the longitudinal axis of the cylindrical body of the cutter. One of ordinary skill in the art would recognize that one or more techniques may be used to achieve the desired workface angle.

In one embodiment, the workface angle may be achieved by varying the rake angle of the cutters. Rake angle, shown as β in FIG. 6, may be defined as the angle subtended between a plane 620 of the cutting face 615 of the cutter 610 and a line 625 parallel to the longitudinal axis of the mill (not shown). In one embodiment, the cutters may be placed at an angle ranging from −5 to −50 degrees to achieve the desired workface angle, from −10 to −35 degrees in another embodiment, and from −30 to −33 degrees in yet another embodiment.

Referring now to FIGS. 7A-B, one technique for varying cutter geometry to have the desired rake angle is to form an axisymmetric cutter 700 having a cylindrical base 702. By cutting cutter 700 on a plane 704 that forms an angle θ with respect to a plane perpendicular to the axis of the insert 700, a top portion 706 is generated, as shown in FIG. 7A-B. When top portion 706 is rotated 180° and re-attached to base 702, it will be canted with respect to base 702 at an angle that is equal to 2θ. Alternatively, cutter 700 having a top portion 706 canted with respect to base 702 may be created by building up top portion 706 of cutter 700 from base 702.

Referring now to FIG. 8, another technique for varying cutter geometry to have the desired rake angle is shown. Cutter 800 has a cylindrical base 802. By grinding or cutting away portion 808 of cutter 800 to form plane 804 having angle θ with respect to a plane perpendicular to the axis of the insert 800, a top portion 806 is generated, as shown in FIG. 8. Cutter 800 may be ground either prior to or post insertion into the cutter pocket (not shown) of the mill. In some embodiments, top portion 806 may optionally include a tapered tip 812 on the side of the cutter 800 to be inserted into the cutter pocket (not shown).

In a particular embodiment, second mill cutters are complimentary to lead mill cutters, i.e., lead mill cutters and second mill cutters have substantially similar orientations with respect to the workface.

In some embodiments, a protective coating may be provided on a portion or all of various mill components of each of the lead mill and second mill, including for example, the cutters, blades, and bodies. In some particular embodiments, the coating may be applied on the cutters prior to insertion and brazing into the cutter pockets. In other embodiments, the coating may be applied to the cutters after the insertion and brazing of the cutters into the cutter pockets. In yet other embodiments, the coating may be applied to all or any portion of the cutters, blades and mill body before or after brazing the cutters. In a specific embodiment, both the lead and second mill may individually have select portions coated. Specifically, the cutters of the second mill may be coated while the entire head of the lead mill may be coated.

In a particular embodiment, a protective coating may provide an increase in hardness and surface lubricity (low coefficient of friction) to the tool surface. Increases in hardness and/or surface lubricity may provide for a reduction in abrasive wear, work hardening, smearing, galling, and/or welding. Examples of coatings suitable for use on the mill components as disclosed herein include an aluminum titanium nitride (AlTiN) coating and an aluminum chrome (AlCr) coating.

Other types of coatings that may provide increases in hardness and/or lubricity that may be used on a mill component as described herein may include titanium nitride, titanium aluminum nitride, titanium carbonitride, aluminum oxide, chromium nitride, aluminum chromium nitride, and chromium carbide. Other embodiments may include silicon based coatings, such as, for example, silicon nitride or silicon titanium nitride. In high heat applications, a coating with a high oxidation temperature or high heat hardness or a coating having aluminum therein may be desirable. Upon oxidation of aluminum, aluminum oxide may serve as a heat barrier. In a particular embodiment, the coating may include metal providing strength to the coating and a lubricious material, such as aluminum, fluoropolymers, including TEFLON® (E.I. DuPont de Nemours Corporation, Wilmington Del.). One of skill in the art would recognize that the selection of protective coating may be dependent upon several factors including, for example, wear resistance, surface lubricity, and oxidation temperature. In one particular embodiment, the protective coating has a static coefficient of friction of about 0.4 or less (against steel).

The coating may be applied by various techniques known in the art, including physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma assisted chemical vapor deposition (PACVD), and plating. One of ordinary skill in the art would recognize that each coat of material may include multiple layers. In applying the coatings, depending on the desired thickness of the coating, the coating may include a single coat (or pass) of material or multiple coats with one or more coating compositions. In one embodiment, the coating may be applied to the lead mill and/or second mill in a total thickness ranging from about 2 to 15 microns. In a particular embodiment, the coating may include two coats of material, each coat ranging from about 3 to 4 microns, to provide a total coating thickness ranging from about 6 to 8 microns.

In one embodiment, the protective coating may have a hardness of at least about 1,000 HV. In another embodiment, the protective coating may have a hardness of at least about 2,000 HV.

As shown in FIG. 1, second mill 130 may be located a selected distance X on the shaft 120 above the lead mill 110. Distance X, as shown in FIG. 1 is measured from the gage 111 of lead mill 110 to the gage 131 of second mill 130. The distance X may be selected to provide sufficient flexibility in shaft 120 between lead mill 110 and second mill 130 during a milling process. The flexibility, which allows for the deflection observed in shaft 120, is dependent upon the characteristics of the shaft 120 and the applied load according to the following relationship:

δ_(max)=(F*L ³)/(3E*I)   (1)

where δ_(max) is the maximum deflection; E is the modulus of elasticity; I is the moment of inertia; F is the total load; and L is the length of the shaft (i.e., the distance X in the above discussion).

For a given set of characteristics of a mill assembly (ID, OD of shaft, material type), the length of the shaft from the lead mill to a tangential engagement or contact point with the casing wall must be sufficient to allow for enough flexibility or deflection in the shaft to mill a window in the casing under the applied load without failure of the milling system. As used herein, the “engagement point” refers to the point on the mill assembly or BHA (or BHA component), i.e., second mill, motor, stabilizer, or drill string, which touches the casing wall, as the mill assembly begins to deflect and mill through the casing. For unit deflection (δ_(max)=1) of a given shaft, the allowed load for a particular mill assembly may be determined by:

F=(3E*I)/L ³   (2).

Thus, the design of a mill assembly, and in particular the distance between a lead mill and second mill or other type of engagement point, may be selected or optimized in accordance with the above relationship to allow for sufficient flexibility under applied loads. In other words, in order to select a location for the second mill, the above relationships may be used based on known values (of inertia and modulus of elasticity). Alternatively, the distance may be selected and a recommended load can be provided. Those having ordinary skill in the art will appreciate that other design considerations may also affect the ultimate placement of the second mill or length of the shaft from the lead mill to the engagement point.

In a particular example, in a 5 inch casing system (duplex 25% chrome casing) using a 2.25 inch OD, a 1.25 inch ID, and a 58 inch long shaft, unit deflection of the shaft may be achieved under an applied load of at least 525 pounds. One of ordinary skill in the art would recognize that for a given mill system, that as the length of the shaft is increased, the applied load that will result in unit deflection may decreased. In another embodiment, in a 5 inch casing system using a 2.25 inch OD, a 1.25 inch ID, and a 180 inch long shaft, unit deflection of the shaft may be achieved under an applied load of at least 17.56 pounds. Further, one of ordinary skill in the art would also recognize that for a given force required for unit deflection, the moment of inertia (f[OD,ID]) and length of the shaft may be varied in accordance with Equation 2 above to result in the same force for unit deflection. Additionally, one of ordinary skill in the art would recognize that there may be a critical load needed for unit deflection after which the mill may fail to cut open a window in the casing and instead cut into the whipstock.

In a particular embodiment, a mill assembly such as the one disclosed herein may be included a one-trip milling/whipstock system, such as those described in U.S. Pat. Nos. 5,771,972, 6,102,123, 6,648,068, which are herein incorporated by reference in their entirety. Briefly, a one trip mill system, as shown in FIGS. 8A and 8B include a milling assembly generally designated as 30 and a whipstock assembly generally designated as 60 that includes a whipstock 44. The mill assembly 30 includes a lead mill generally designated as 32, which is attached to the bottom end of a shank or shaft 31. Located above and spaced from the lead mill 32 is, for example, a second or follow mill 33 that is also mounted to the shaft 31. The upper end of the shaft 31 is either threadably connected to a drill string or threaded to another subassembly (not shown). A tubular member 27 may form the shaft 31 on which mills 32 and 33 are mounted. Tubular member 27 may include a lower reduced diameter portion on which mill 32 is disposed with mill 33 being disposed on the fill diameter of tubular member 27. This reduction in diameter provides flexibility between mills 32 and 33 during the milling process.

Blade 38 immediately adjacent the parallel surface 45 of whipstock 44 may be sufficiently wide to accommodate the shear bolt 39 threaded into the blade 38. The head of the shear bolt 39 is seated in the top of the whipstock 44 and the shank 54 of the shear bolt 39 is threaded into blade 38. The shank 54 may be hollow so that, once the bolt 39 is sheared, the shank 54 serves as a nozzle extension for nozzles 69 positioned at the base of shank 54 and at the entrance to flex conduit 37 that directs fluid to the whipstock anchor (not shown).

The whipstock 44 has a diameter D_(W) that approximates the inside diameter D_(I) of the interior wall of casing 11 which allows whipstock 44 to be lowered through cased borehole 9. Whipstock 44 also includes a profiled ramp surface 28 having a curved or arcuate cross section and multiple surfaces, each of the multiple surfaces forming its own angle with the axis 26 of whipstock 44. Profiled ramp surface 28 includes a starter surface 45 having a steep angle preferably 15°, a vertical surface 46 preferably parallel to the axis 26, an initial ramp surface 47 having a standard angle ranging from about 0.5 to 3°, a “kick out” surface 48 having a steep angle preferably 15°, and a subsequent ramp surface 49 having a standard angle ranging from about 0.5 to 3°. It should be appreciated that these angles may vary. For example, the starter ramp surface 45 may have an angle A in the range of 1 to 45° in one embodiment, 2 to 30° in another embodiment, 3 to 15° in yet another embodiment, and about 15° in still another embodiment. The vertical surface 46 may have a length approximately equal to or greater than the distance between mills 32 and 33. In a particular embodiment, ramp surfaces 46, 49 may range from greater than zero to 15°. One of ordinary skill in the art would recognize that the surfaces angles may be selected depending on the desired window dimensions.

The backside 62 of the whipstock 44, especially adjacent the upper end of the whipstock 44, is contoured to conform to the inside diameter D_(I) of the interior wall of the pipe casing 11 for stability of the top of the whipstock 44. The opposite lower end of the whipstock 44 is secured to, for example, a hydraulically actuated anchor (not shown). A typical anchor is shown in U.S. Pat. No. 5,657,820, incorporated herein by reference in its entirety.

The mill 32 and whipstock 44 disclosed herein are configured such that the mill 32 tends to cut the wall of the casing 11 and not the whipstock 44. To achieve this objective, various factors are taken into consideration including the contact area and contact stress between the mill 32, casing 11, and whipstock 44 and the cutability of the metal of the casing and of the metal used for the whipstock 44.

Advantageously, embodiments disclosed herein may provide for at least one of the following. Mill assemblies incorporating cylindrical cutters on each of the lead and second mill may allow increased mill efficiency and mill life for ease in modification of existing mill assemblies. By forming a mill assembly that has a second mill distance from the lead mill that is selected to allow for flexibility, a mill assembly as disclosed herein may be effective in cutting a window in a casing that would otherwise be unobtainable, without otherwise altering the mill and drill assembly components.

When milling through a casing, one potential mode of failure of the mill is by galling and welding observed at the mill face, especially when milling through a chrome casing. The inclusion of a protective coating on the cutting structure may prevent or reduce such occurrences of galling. In the various embodiments that may include a lubricious material such as aluminum or silicon in the protective coating, the coating may also have an increased life span due to the preferential oxidation of aluminum, which further reduces galling and welding and contact temperatures.

Additionally, by coating the cutting structure, a dramatic increase in wear resistance of the mill may be observed. The mill assemblies disclosed herein may endure significant increases in downhole life as compared to a typical mill assembly, and even when tripped, mill assemblies made in accordance with embodiments disclosed herein, and specifically the gage of the mills disclosed herein, may have worn minimally while downhole.

Furthermore, as the cutting structure meets the casing wall, the cutting structure typically encounters severe vibrations that frequently lead to cracks in the cutters. By varying the cutter geometry and/or placement of cutters with respect to the workface surface, i.e., interior casing surface, the incidence of cracking may be decreased and the cutting efficiency, and thus mill life, may be increased. Additionally, by varying the cutter geometry by grinding or cutting the cutters to have the desired rake and workface angles, the number of cutters on a particular mill may be increased, and thus the amount of wear each cutter experiences may decrease.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

1. A mill assembly, comprising: a shaft; a lead mill secured to a first end of the shaft, the lead mill comprising: a first body having a plurality of first blades; and a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades; and a second mill secured to the shaft a selected distance from the lead mill, the second mill comprising: a second body having a plurality of second blades; and a plurality of second cutters having substantially cylindrical bodies secured to the plurality of blades.
 2. The mill assembly of claim 1, wherein a gage diameter of the second mill is substantially the same as a gage diameter of the lead mill.
 3. The mill assembly of claim 1, wherein the plurality of second cutters are complimentary to the plurality of first cutters.
 4. The mill assembly of claim 1, wherein at least a portion of at least one of the plurality of first and/or second cutters have a protective coating thereon.
 5. The mill assembly of claim 1, wherein at least a portion of at least one of the plurality of first and/or second blades have a protective coating thereon.
 6. The mill assembly of claim 1, wherein at least a portion of at least one of the first and/or second body have a protective coating thereon.
 7. The mill assembly of claim 1, wherein at least a portion of at least one of the plurality of first and/or second cutters, the plurality of first and/or second blades, and the first and/or second body have a protective coating thereon, and wherein the protective coating comprises at least one of AlTiN, AlCr, SiTiN, and SiN.
 8. (canceled)
 9. The mill assembly of claim 1, wherein at least a portion of at least one of the plurality of first and/or second cutters, the plurality of first and/or second blades, and the first and/or second body have a protective coating thereon, and wherein the coating has a thickness ranging from about 6 to 8 microns.
 10. The mill assembly of claim 1, wherein at least a portion of at least one of the plurality of first and/or second cutters, the plurality of first and/or second blades, and the first and/or second body have a protective coating thereon, and wherein the coating has a hardness of at least 1000 HV.
 11. The mill assembly of claim 1, wherein at least one of the plurality of first cutters and second cutters has a workface angle ranging from about −5 to −40 degrees.
 12. (canceled)
 13. The mill assembly of claim 11, wherein at least one of the plurality of first cutters and second cutters has a workface angle of about −15 to −18 degrees.
 14. The mill assembly of claim 1, wherein the distance between a gage of the lead mill and a gage of the second mill is selected based upon the relationship: F=(3E*I* δ_(max))/ L³ wherein F is load; E is modulus of elasticity of the shaft; I is the moment of inertia; δ_(max) is the maximum deflection of the shaft; and L is the length of the shaft from a gage of the lead mill to the second mill.
 15. A method of milling a window in a tubular in a wellbore, comprising: engaging a lead mill of a mill assembly against an interior surface of the tubular, the lead mill secured to an end of a shaft and comprising: a first body having a plurality of first blades; and a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades; rotating the mill assembly; moving the mill assembly along a surface of a whipstock assembly as the lead mill cuts the window in the tubular, thereby deflecting the lead mill and shaft outwardly through the window in the tubular; and engaging a second mill of the mill assembly against the window in the tubular, the second mill secured to the shaft a selected distance from the lead mill and comprising: a second body having a plurality of second blades; and a plurality of second cutters having substantially cylindrical bodies secured to the plurality of blades.
 16. The method of claim 15, wherein the lead mill engages the tubular such that at least one of the plurality of first cutters form a workface angle with the tubular ranging from about −5 to −40 degrees.
 17. The method of claim 15, wherein the second mill engages the tubular such that at least one of the plurality of second cutters form a workface angle with the liner ranging from about −5 to −40 degrees.
 18. The method of claim 15, wherein a gage diameter of the second mill is substantially the same as a gage diameter of the lead mill.
 19. The method of claim 15, wherein at least a portion of at least one of the plurality of first and/or second cutters, the plurality of first and/or second blades, and the first and/or second body comprise a protective coating.
 20. A method of designing a mill assembly, comprising: determining characteristics of a lead mill, the lead mill comprising: a first body having a plurality of first blades; and a plurality of first cutters having substantially cylindrical bodies secured to the plurality of first blades; determining characteristics of a shaft having a first and second end, wherein the first end is adapted to receive the lead mill and the second end is adapted to be threadably connected to a drill assembly; determining characteristics of an engagement point; and selecting a location on the shaft for the engagement point to be placed.
 21. The method of claim 20, wherein the location is selected based upon the relationship: F=(3E*I* δ_(max))/ L³ wherein F is load; E is modulus of elasticity of the shaft; I is the moment of inertia; δ_(max) is the maximum deflection of the shaft; and L is the length of the shaft from a gage of the lead mill to the engagement point.
 22. The method of claim 20, wherein the engagement point is selected from the group of a second mill, a stabilizer, a motor, and drill string.
 23. A mill assembly, comprising: a shaft; a lead mill secured to a first end of the shaft, the lead mill comprising: a body having a plurality of blades; and a plurality of cutters having substantially cylindrical bodies secured to the plurality of blades; and a protective coating disposed on at least a portion of at least one of the body, the plurality of blades, and the plurality of cutters.
 24. The mill assembly of claim 23, further comprising: at least one second mill secured to the shaft a selected distance from the lead mill.
 25. The mill assembly of claim 23, wherein at least a portion of the second mill comprises a protective coating thereon.
 26. The mill assembly of claim 23, further comprising: at least one of a motor and a stabilizer secured to the shaft a selected distance from the lead mill.
 27. The mill assembly of claim 23, wherein the protective coating comprises at least one of AlTiN, AlCr, SiTiN, and SiN. 